Tips for diagnosing the performance issues of RV air conditioners.
By Bill Hendrix, F761S
At this time of year, many of us are traveling in climates where we depend on our RV air conditioners to keep us cool and comfortable. Several steps can be taken to determine whether an RV air conditioner is delivering the appropriate amount of cold air. However, in so doing, we also must make a few assumptions: that the air conditioner has been installed per the installation instructions, that the distribution ductwork is properly sized with a static pressure within the design specs, and that the return air path has the appropriate opening.
You may be thinking that these are big assumptions, but all responsible motorhome manufacturers collaborate with the air-conditioner supplier to ensure good installations. Shorting any of the installation requirements may result in poor airflow and reduced cooling, and could promote evaporator freeze-ups. It is in the best interest of the RV manufacturer to conform to the installation specifications, as trying to retrofit wiring or ductwork changes would be extremely difficult, if not impossible, and very expensive.
First and foremost, you need to know what you are working on. To accomplish this, the most reliable source of information is the data plate (or serial number plate) on the unit. This will tell you the make, the model, the serial number, the electrical amperage of the fan, and the amperage of the compressor “” at a specified voltage, in a specified temperature, at a specified humidity. Data plates may not reveal all these specifications, but at a minimum they should state the electrical specs. In the absence of the environmental specs, it would be safe to apply 95 degrees Fahrenheit ambient temperature and 50 percent relative humidity, but more on this a little later.
Prior to conducting any tests, go through the basics. For a proper evaluation, you will need a relatively warm day “” not less than 70 degrees Fahrenheit “” with the unit running at maximum cooling on high-speed fan and all the registers fully open for at least 15 or 20 minutes to get the ductwork temperature and the refrigerant operating pressures stabilized. Be sure the filter is clean and made of the correct filter material. If not, remove the filter for the test.
One of the more important preparation issues is to ensure that all of the cold air is being discharged into the living compartment. To do so, inspect the connection between the transition duct and the distribution duct (or ceiling assembly). The transition duct is the short, square vertical duct between the roof unit and the distribution duct (or ceiling assembly). With the air conditioner running, inspect for cold air escaping and recirculating to the evaporator. This can be done by feeling around in the return air cavity. There will be air movement but you should not detect any cold air escaping in this area. Apply heat-resistant silver metallic duct tape (not cloth) to any area where cold air leakage is present. Some installations may be very difficult to access, but if leakage is discovered, it must be corrected. It may even be necessary to cut an access to the transition duct through the bottom of the distribution duct and silver-tape the leakage from the inside. Then repair the access and silver-tape the hole; however, I would leave that job for a service center. Silver-taping air leakage is most effective when done to the inside of the discharge air duct and to the outside of a return air duct. Never tape the inside of a return duct on a basement air conditioner if it is between the filter and the blower. If the tape comes loose, it would be drawn into the blower.
The first test is very simple and requires only a single probe-type thermometer. However, make sure there is no recirculation of the cold air as described above or you will not get a valid temperature test. Probe-type thermometers are inexpensive and are available at most hardware and auto supply stores. The reason for using only one thermometer is that it eliminates the chance that a thermometer may be a little off. With only one, we look just for the temperature difference, so accuracy is not an issue.
With the air conditioner running on maximum cooling and high-fan speed and all the distribution registers fully open, place the probe of the thermometer in one of the discharge vents that has good air volume. Hold the thermometer in place a few minutes until the needle stabilizes and note that temperature. Use the same thermometer and place it at the return air inlet (where the filter is). Allow it to stabilize and then note that temperature. Subtract the discharge temperature from the return temperature; this is the Delta T (temperature differential or temperature split).
The air conditioner is designed to reduce the temperature of the air 20 degrees Fahrenheit as it flows through the evaporator coil at 50 percent relative humidity. The actual temperature coming out of the registers is not the criteria, but the amount of temperature drop is the factor. If the return air is 100 degrees Fahrenheit, we would expect the discharge air to be around 80 degrees Fahrenheit, but the room temperature would be gradually lowered to the thermostat setting. If the humidity is very low, the temperature drop may be 23 degrees or 24 degrees. If the humidity is very high, the number may be 17 degrees or 16 degrees. When the humidity is high, the evaporator collects more moisture from the air and discharges this moisture to the outside. This cold condensation steals some cooling effect from the air.
If the air conditioner passes this test, you are through. If the Delta T is significantly low, the air conditioner is not cooling properly and you must look for the problem. Be aware that the humidity level inside the coach may be significantly different from the outside humidity levels. We cook, bathe, and breathe, all of which contributes to a higher indoor humidity, so take this into account when evaluating the Delta T.
Checking for airflow restrictions
The next step would be to make sure there are no restrictions in the ductwork. Seldom will all the discharge registers have the same volume of airflow, but if there is virtually no air volume at any of the outlets, further investigation is necessary. Remove a near and far outlet register, and use a flashlight at one end and an inspection mirror at the other end to ensure clear passage. If you discover an obstruction or a collapsed duct and an easy remedy is not apparent, it may be necessary to install another vent at the problem area in order to gain access and effect a repair. Again, this type of repair should be done by a service center.
Verifying electrical specifications
Testing the electrical specifications will require some technical ability and a comfort level with a clamp meter. If you are not proficient in this area, take the unit to a service center for electrical diagnosis. For those technically inclined, proceed with the amperage test.
With the air conditioner still running on maximum cooling and high-speed fan, set the clamp meter for the appropriate 120-volt-AC range, and use the test leads to probe any convenient electrical outlet. To make a valid amperage test, the AC voltage should be no less than 110 volts.
Turn off the air conditioner by tripping the appropriate breaker at the electrical panel. Locate the air conditioner’s electrical junction box, normally found in the 14-inch-by-14-inch ceiling opening directly under the roof unit. Some coaches do not have the 14-inch-by-14-inch opening, so the junction box will be located elsewhere. Remove the junction box cover and pull out the black and the white wires. Tighten the wire nuts to ensure a solid electrical connection and capture the black wire only with the clamp meter; reset the breaker to start the air conditioner. Set the thermostat on a low temperature and high fan speed.
(1) If the unit has a compressor time delay, the fan will start first, and the clamp meter will read only the fan’s amperage. Make a note of this amperage reading. In about 2 to 3 minutes the compressor will start and the meter reading will begin to increase. After several minutes the meter will stabilize, showing the amperage of both the fan and the compressor. Make a note of this reading. Subtract the fan number from the second reading to arrive at the compressor amperage.
(2) If the unit has no compressor time delay, the fan and the compressor will start at the same time. Let the unit run several minutes until the meter stabilizes to show the amperage for both the fan and the compressor. Note this number. Next, increase the thermostat temperature setting until the compressor turns off. The meter will show just the fan amperage. The difference between the two readings will be the compressor amperage.
Trip the breaker again to stop the air conditioner, remove the clamp meter, reassemble the junction box, and reset the breaker. We now have a valid amperage number for the fan and for the compressor.
Let’s say the data plate states the fan to be 2.7 amps and the compressor to be 12.3 amps for a total of 15 amps at 115 volts. Compare that to the amperage numbers you checked to see how close they are. Seldom will the numbers be right on the nose, but they should be in close proximity. You will also see some small fluctuation in the amperage numbers as you take the readings, but they will establish a close range, and you can take the middle of this range as a good number.
To be very precise in our evaluation, we can adjust the compressor amperage for the present temperature conditions as follows. To the data plate compressor amperage, add 0.5 amps for each 5 degrees Fahrenheit the outside temperature is above 95 degrees or subtract 0.5 amps for each 5 degrees the outside temperature is below 95 degrees. This will reflect the fact that when it is very hot outside, the compressor will consume another 0.5 amp for each 5 degrees Fahrenheit the temperature is above 95 degrees and, conversely, will consume 0.5 amp less for each 5 degrees Fahrenheit when the temperature is below 95 degrees.
Let’s presume the available incoming voltage is 115 volts AC and the outside temperature is 85 degrees Fahrenheit. We would reduce the data plate compressor amperage number by 1.0 amps for the 85-degree ambient (10 degrees cooler), so in this particular example, the compressor amperage would be 12.3 amps reduced by 1.0 amps to equal 11.3 amps “” the adjusted amperage for the compressor at that temperature. If the adjusted compressor amperage is within 1 amp and the temperature drop is appropriate, the air conditioner is performing to design specifications.
If the compressor amperage is inordinately low, the Delta T is low, and the air delivery is good, we would suspect a low refrigerant charge or a failing compressor. If the unit has lost refrigerant, an oily spot will normally appear somewhere along the tubing or at one of the coils. If the compressor amperage is inordinately high, we would suspect a blockage in the refrigerant circulation or an internal problem with the compressor. Resolving problems within the refrigerant system requires specialized equipment and a trained technician. Sometimes the cost of repair will approach or exceed the cost of replacement, so get an estimate and compare that with the cost of a replacement air conditioner. I personally don’t like to spend major dollars on appliances that are more than 5 years old.
In most cases, the fan will either work or it won’t work, and fan amperage should be very close. If the fan amperage is too high, it should be replaced. If all of the numbers look on target but the temperature drop is low, inspect the condenser and evaporator coils for debris. If the air conditioner has been operating without a filter, a dirty evaporator coil would cause performance problems, but this should also be evident in poor airflow. If the condenser has accumulated trash, it won’t be able to discharge the proper amount of heat, and the evaporator temperature will not respond accordingly, but this may show up only in warmer ambients.
We sometimes get the impression that the bedroom air conditioner works better than the one in the living area. Please realize that there is much more heat gain in the living area, especially if there are slideouts, a picture window, and a large windshield. Compare that to the small bedroom, with a much more limited heat gain.
If you are not comfortable doing any of the above, take the coach to a service center for professional help.
Performing these checks should help to keep you and your air conditioners cool.